Okay, let's tackle this question about the investigation used for detecting and confirming HIV. First, I need to recall the standard diagnostic tests for HIV. The core concept here is understanding the different stages of HIV testing and which tests are used for screening versus confirmation.

HIV testing typically starts with a screening test, like an ELISA or rapid antigen/antibody test. These detect antibodies or antigens. If the screening is positive, a confirmatory test is needed. The gold standard for confirmation is the Western blot or, more recently, the HIV-1/HIV-2 differentiation immunoassay. But wait, in some settings, PCR for viral load might be used to confirm in early stages, especially in infants or when there's a window period. However, the question is about detection and confirmation. So the confirmatory test after a positive screening is usually Western blot. However, in some places, they might use a nucleic acid test (NAT) for confirmation. But I think the traditional confirmatory test is Western blot. Let me check my notes. Yes, Western blot is the confirmatory test. So the correct answer is likely the Western blot. The options might include ELISA, Western blot, PCR, and maybe something else like rapid test. The options are missing here, but assuming the correct answer is Western blot. Let me structure the explanation accordingly.

Now, the core concept is the algorithm for HIV testing. Screening with antibody or antigen tests, then confirmation with Western blot or molecular tests. The correct answer is the confirmatory test. For the incorrect options, ELISA is a screening test, PCR detects viral load but isn't used for initial confirmation, and rapid tests are also screening. The clinical pearl is to remember the two-step process: screen with ELISA or rapid, confirm with Western blot or NAT. Also, in some cases, PCR is used for early detection in neonates. So the correct answer here is the Western blot.

**Core Concept**
HIV diagnosis follows a two-step process: initial screening (e.g., ELISA) to detect antibodies or antigens, followed by confirmatory testing (e.g., Western blot or nucleic acid amplification test) to verify infection. Confirmatory tests target specific viral proteins or genetic material to avoid false positives.

**Why the Correct Answer is Right**
The **Western blot** is the traditional confirmatory test after a reactive screening test. It detects specific HIV proteins (gp120, gp41, p24) using antibody binding, ensuring high specificity. For acute infections or infants, **nucleic acid testing (NAT)** may confirm HIV by detecting viral RNA/DNA directly, bypassing antibody detection. Both methods resolve equivocal cases but serve different clinical contexts.

**Why Each Wrong Option is Incorrect**
**Option A:** *ELISA (Enzyme-Linked Immunosorbent Assay)* is a screening test, not confirmatory. False positives are common due to cross-reactivity with other antibodies.
**Option B:** *Rapid antigen/antibody tests* detect p24 antigen and antibodies but lack the specificity of confirmatory methods.
**Option C:** *PCR for viral load* measures active replication but isn’t used for initial diagnosis; it’s reserved for monitoring treatment efficacy.

**Clinical Pearl / High-Yield Fact**
Never rely on a single test for HIV diagnosis.

✓ Correct Answer: B. Reverse Transcriptase-PCR